Process for electrolytic deposit of tin



Patented July 29;, 1941 t g v UNITED STATES PATENT OFFICE Roger T.Robinson, Huntington Park, Calif.

No Drawing. Application January 4, 1938,. a v,

Serial No. 183,310

8' Claims. (oi.2o4' '122) This invention relates to a process'of'electhe tin of the solution of the cell at least twotrolyticallydepositing tin from, solution. The thirds in thestannic form. A processmay be used for the recoveryof tin from r The process of thepresentinvention is useful solutions used in leaching tin either from tin inthe depositing of tin fromsolutions containcans or recovering ,tin fromsolutions used for 5 ing notonly tin but'alsoiron. Whenelectrolyzleaching tin from other sources. It also may be ing tin from asolution containing iron, it is found used asapart of the process ofelectrolytically that the iron in the solution remains nearly refiningtin where tin is employed, for example, completely inthe ferrous,condition until the .tin as the anode of a, cell. of the solution hasnearly all beenoxidized to Heretofore, in" the electrolytic deposit oftin m the stannic condition, after which time the iron from solutionconsiderable difficulty has been in solution startsto be converted tothe ferric experienced in securing 'a good smooth finecondition. For theoperation of the process of grained deposit of tin on the cathode of thecell. 8 the present invention, it is found of consider- In order tosecure an adherent fine-grained,-unlable value to maintain the iron insolution in form deposit of tin on the cathode, 'it has been 15substantially the ferrous condition. A solution. usually found necessaryto operate on'relatively containing iron in the ferric form attacks veryexpensive solutions. rapidly any iron present, and since it is desir-The present invention has forits general obable to forma portion atleast of theapparatus ject to provide a process for depositing tinfromwith iron, a solution containing the ferric ion solution which willresult in an adherent, flnego is to be avoided.

grained, uniform deposit of tin on the cathode Another important featureof the present inof the cell from an inexpensive solution which ventionconsists in the discovery that by the addimay be easily controlled. tionof a small part of glue to the solution elec- The present inventionembodies a number of trolyzed marked improvements may beeffecteddiscoveries with respect to the process of 'de- 25 in thecharacter of the deposit of tin secured. positing tin in electrolyticcells, these discoveries By the addition of a small part of glue, thederesiding mainly in the oxidation .versus reduction posit becomessmoother and more consolidated.

- condition of the tin ion in solution, the effect of The presentinvention, together with various glue on the deposit, as well as theconcentration further objects and advantages thereof, will bestv andeffects of other ingredients present. :m be understood from adescription of a preferred I have discovered that the oxidation versusreform or example of depositing tin in accordance duction condition ofthe tin ion present in an acid with the present invention. I have,therefore, solution has a controlling influence on the charhereinafterdescribed a preferred form or examacter of the tin deposit which will beformed. For ple of the process of depositing tin embodying example, ifone seeks to deposit tin electrolytical- 3 the present invention.

1y from a hydrochloric acid solution of tin, the The invention may taketwo forms: one, following phenomena will be observed: wherein theinvention is applied to recovering tin Starting, first, with an acidsolution containfrom a solution used for leaching tin from variing thetin in the stannous condition and elecous sources, and the other beingwhere theprocess trolytically depositing the tin on the. cathode 4 isapplied for the purpose of electrolytically re-' of the cell, the firstdeposit formed will consist fining tin. In the first case, the solution,for exvery largely of needles and fern-like growths ample, from aprocess of leaching the tin plate upon the cathode, and this form ofdeposit, which from used tin cans, is fed continuously into an is quiteunsatisfactory in the process, will conelectrolytic cell, wherein thesolution is. maintinue until approximately one-half to two-thirds r,tained under vigorous mechanical agitation. of the tin remaining insolution has reached the Vigorous mechanical agitation is maintained sostannic condi ion- At h period in the process, as to bring the solutionenteringv the cell which there is a rather abr p h n e and the depositmay contain a. high content of stannous ion of tin from the solutionproceeds generally with quickly to a solution containing most of the tinthe formation of smaller shorter crystals which 50 in the stanniccondition through rapid and improduce a finer-grained deposit. Inaccordance mediate admixture with the bulk of the solution withtheprocess of the present invention, therein the cell. There is therebyavoided the formafore, to obtain electrolytically a good, smooth, tionof an undesirable form of deposit of the tin. dense and fine-graineddeposit of tin, it is de- A further advantage' of vigorous mechanicalagisirable to maintain at all times in the process 53 tation is that itlowers the cell voltage. The solution fed to the cell may contain thetin mainly in the stannous condition.

Temperatures within the range of 60 to 100 F.

are satisfactory in operation, but I prefer to employ a temperature ofaround 70 F. for the electrolytic operation. The lower temperatures havebeen found to give a smoother deposit of tin.

The amount of glue maintained in the solution does not appear to behighly critical. I have found entirely satisfactory results to beobtained with five parts of glue per million, and have increased theconcentration of glue up to and including 1,000 parts per million. Iftoo much glue is used, some of it may separate as a gelatinousprecipitate. Such a precipitate has not been observed at gluecpncentrations of 100 parts per million or less.

The over-all electrolytic reaction takingpla'ce in thecell is theconversion of stannous to stannic ions, with the deposit of tin inaccordance with the following typical equation:

2SnClz- Sn+SnCh Stannous Tin Stannic chloride chloride In the preferredform of the process, the solution within the electrolytic cell ismaintained at nearly the point of completion of the above reaction; thatis to say, the solution is preferably maintained only slightly short ofthe point where all of the tin present is in the stannic form. However,satisfactory deposits may be obtained whenever at least two-thirds ofthe tin present in the solution is in the stannic form. .The solutionentering the cell may as a rule contain the tin, principally in thestannous condition, and the solution within the cell is maintainedprincipally in the stannic condition by proper rapid mechanicalagitation of the entering solution with the solution of the cell. Innormal operation there is withdrawn from the cell solutioncorresponding. substantially in amount to that of the entering solution.If the solution within the cell itself is permitted to contain more thanabout one-third of its tin in the stannous form, it has been foundsubstantially impossible to avoid the formation by the tin beingdeposited of undesirable fernlike or needle growths.

The quantity of tin within the solution is of relatively minorconsequence. I prefer to employ, for example, a solution containingabout 0.12

I have operated satisfactorily with solutions containing as low as 0.006molal parts of stannic chloride, and as high. as 0.24 molal parts ofstannic-chloride. Increase of the concentration of I molal parts ofstannic chloride perliter, although stannic chloride present decreasesthe net rate at which tin is deposited from solution for a given cathodecurrent density. For example, at a cathode current density of 20 amperesper square foot, using a 0.06 molal solution with respect to stannicchloride, I have deposited 0.0022 gram -per square inch per minute oftin, whereas, using a solution 0.24 molal with respect to stannicchloride under similar conditions, deposited only 0.0004 gram per squareinch of tin. The disadvantage of using a lower concentration of stannicchloride than about 0.06 molal and more preferably 0.12 molal is thatthe maximum permissible cathode current density is thereby decreased.The cell may gas or produce loose deposits if high current densities areused with too low a concentration of stannic chloride present I preferthat the solution should also contain excess acid. As a rule,hydrochloric acid about 1.9 molal in strength has been used. Differentacid concentrations may be used, and I have varied the hydrochloric acidconcentration from to 4 molal. The higher the concentration of acid, thehigher the current density for a given voltage on the cell. On the otherhand, increasing the acid concentration decreases the rate of depositwith a given current density.

I have found that the current density usable in the process may varywidely, and if the remaining conditions of the process are adhered tothe current density is not a particularly critical factor. I have usedcurrent densities, for example, of between 2.2 and 46 amperes per squarefoot. In general, as long as hydrogen is not evolved, an increase ofcathode current density will result in an increase of cathode currentefficiency. Hence, it is usually desirable to operate with a cathodecurrent density as high as possible without causing the cell to evolvehydrogen. Except at low concentration of such, such as .006 molal, animportant controlling factor of the process appears to be the actualvoltage applied across the cell. found that a cell having a graphiteanode, a tin cathode spaced one inch from the anode and having theelectrolyte rapidly circulated commences to gas and the resultingdeposit ceases to remain in the smooth fine-grained, tightly adheringcondition desired. The cell operating voltage is preferably kept belowabout 1.7 and at some suitable value higher than the no-current voltageof the cell, which is about one volt, and any desired current densityconsistent with these voltages and other factors of the process may beused satisfactorily. As high a current density should be used as ispossible without the evolution of hydrogen. The higher the currentdensity, the greater the cathode current efficiency. At extremely lowconcentrations of stannic chloride in solution, such as 0.006 molal,voltages below 1.1 have been used. These are employed in treatingclean-up solutions.

The solution preferably contains ferrous chloride, but not ferricchloride. I prefer to maintain the concentration of ferrous chloride inthe electrolytic cell at about 0.27 molal. However, I have varied theferrous chloride concentration from about 0.03 molal to about 0.54molal. With the lower concentrations of ferrous chloride the currentdensity in the cell is lower for a' given voltage than at the higherconcentrations of ferple, if it is desired to electrolytically refinetin,

the anode of the cell may be composed of the tin which it is desired torefine and the same Above about 1.7 to 1.8 volts I have conditions withrespect to the solution within the cell maintained as before given,except that it may be desirable to maintain more of the tin in thestannous condition than was indicated in trolytic refining of tin inwhich rapid solution of tin by the cell solution is not necessarilydesirable, it may be advantageous to increase the relative concentrationof stannous ion, since such an increase results in an improvement ofcathode current efiiciency. For this reason, it may be desirable tooperate the cell with as high a relative concentration of stannous ionas is possible and at the same time obtain a tin deposit of satisfactorycharacter. The control of the relative oxidation conditions between thestannous and stannic ions present may be effected by varying the area ofthe tin of the anodes, or if the solution rate of the tin of the anodesis insuflicient the solution may be circulated from the cell over thetin to be refined and returned to the cell.

In any case, in the electrolytic deposition of tin, according to theprocess of this invention, the major important factors to preserve are:

(1) That the tin present in the solution should be at least two-thirdsin the stannic condition;

- (2) That a small portion of suitable addition agent, such as glue, bepresent;

(3) That the solution be acid;

half and all of the tin present in the stannic form whereby the iron ispresent substantially entirely in the ferrous form, the voltage appliedto the electrolytic action being below about 1.7.

4. A process for producing tin, which comprises subjecting a solution toelectrolysis for the deposit of tin, said solution comprising anhydrochloric acid solution of tin and iron, the oxidation condition ofthe solution throughout the electrolytic action during the electrolyticaction being maintained at a point corresponding to nearly 100% of thetin being present in the stannic condition butnot beyond suchoxidationcondition as to retain the iron present in the ferrous state,the solution undergoing electrolytic action containing a small quantityof glue.

5. A process for producing tin, which comprises subjecting toelectrolytic action an hydrochloric acid solution of tin, such solutionbeing between 0.006 and 0.24 molal with respect to tin, from 0.03 to0.54 molal with respect to iron, and having an oxidation conditionthroughout the electrolytic action corresponding to between onehalf andall of the tin present in the stannic form whereby the iron is presentsubstantially entirely in the ferrous form, the solution under-"- goingelectrolytic action containing a small quantity of glue.

(4) That the current density be limited so that hydrogen is not evolvedin the cell.

While the particular form of the process herein described is welladapted to carry out the objects of the invention, it is to beunderstood that various modifications and changes may be made, allwithout departing from the scope of the invention as defined in theappended claims.

I claim:

1. A process for producing tin, which comprises subjecting a solution toelectrolysis for the deposit of tin, said solution comprising anbydrochloric acid solution of tin and iron the oxidation condition ofthe solution during the elec-,- trolytic action being at all timesmaintained at a point corresponding to'nearly 100% of .the tin presentbeing in the stannic condition but not beyond such oxidation conditionas to retain the iron present in the ferrous state.

2. A process for producing tin, which comprises subjecting toelectrolytic action an hydro-- chloric acid solution of tin, suchsolution being between 0.006 and 0.24 molal with respect to tin, from0.03 to 0.54 molal with respect to iron, and having throughout theelectrolytic action an oxi- 6. A process for producing tin, whichcomprises subiecting to electrolytic action an hydrochloric acidsolution of tin, such solution being between 0.006 and 0.24 molal withrespect to tin, from 0.03 to 0.54 molal with respect to iron, and havingan oxidation condition throughout the electrolytic action correspondingto between onehalf and all of the tin present in the stannic formwhereby the iron is present substantially entirely in the ferrous form,the voltage applied to the electrolytic action being below about 1.7,the solution undergoing electrolytic action containing a small quantityof glue.

dation condition corresponding to between onehalf and all of the tinpresent in the stannic form whereby the iron is present substantiallyentirely in the ferrous form. a

3. A process for producing tin, which comprises subjecting toelectrolytic action an hydrochloric acid solution of tin, such solutionbeing between 0.006 to 0.24 molal with respect to tin, from 0.03 to 0.54molal with respect to iron, and having throughout the electrolyticaction an oxidation condition corresponding to between one- 7. A processof electrolytically producing tin,

which comprises subjecting to electrolytic action a supply ofhydrochloric acid solution of tin, such solution containing iron andhaving an oxidation condition throughout the electrolytic actioncorresponding to between one-half and all of the tin present being inthe stannicform whereby the iron is present in the ferrous form,continuously feeding to the supply of solution undergoing electrolyticaction a fresh supply of solution containing tin mainly in the stannouscondition, subjecting the supply of solution and the feed solution tovigorous agitation, and continuously withdrawing solution from the cell.

8. A process for producing tin, which comprises subjecting a solution toelectrolysis for the deposit of tin, said solution comprising ahydrochloric acid solution of tin and iron, from 0.03

to 0.54 molal with respect to iron, the oxidation condition of the.solution during the electrolytic action being at all timesmaintainedat' a point corresponding to between one-half and all of the tin presentbeing in the stannic form but not beyond such oxidation condition so asto retain the iron present in the ferrous state.

ROGER T. non'msou.

